Proton transfer is one of the most fundamental processes in nature. Proton transfer is involved in numerous chemical reactions, biological functions and material properties. Photoacids are molecules that undergo proton (H+) dissociation upon irradiation, and may be defined as any compound convertible into a strong acid by photolysis. Photoacids promise spatial and temporal control of these processes in a noncontact way, and can also provide a way to convert photoenergy into other types of energy (e.g., electrical energy). As used herein, the term “photoacid” refers to the molecules that reversibly undergo proton photodissociation and thermal reassociation, under photo-irradiation and in the dark, respectively. The reassociation may also be induced by photo-irradiation at a wavelength different from the wavelength that induces proton dissociation.
Photoacids have been studied for several decades, and several reviews on photoacids have been published. Photoacids have been used to study molecular proton transfer, and have been exploited to control molecular and supramolecular events. However, the photo-induced proton concentration of reported photoacids has not been sufficient to drive or control many acid catalyzed or pH-sensitive processes, so that the potential of photoacids has yet to be realized. For example, catalyzing reactions using photoacids was first proposed in the 1970s and is still described as a potential application in some recent published papers.
A major challenge using photoacids for processes involving proton transfer is that the lifetime of the proton-dissociation state is limited by the lifetime of the conjugated base of the photoacid in the ground state. The short lifetime of the proton dissociation state is useful for studying some fast processes. However, the liberated proton does not have enough time to diffuse away from its counter ion and thus cannot catalyze a chemical process or significantly alter a macroscopic property even though the theoretical excited-state acid dissociation constant (pKa*) can be very low. One recent work reported a photoacid with a relaxation time of the proton dissociation state close to 1 second, which was estimated by pump-probe absorption spectroscopy.